Physiological data, such as electrocardiographic (ECG) data, often is transmitted to a centralized location for processing and analysis. Such transmitted data, for example, may be supplied directly to a computer for computer analysis thereof. For real-time analysis, telephone transmission of the ECG waveforms from the doctor's office to the computer provides the necessary speed of transmission. Presently, the most commonly used method of transmitting ECG waveforms by telephone transmission is by use of FM subcarriers. (See, for example, U.S. Pat. No. 3,199,508). If these systems are well designed and properly maintained, waveforms of reasonable quality can be transmitted. However, those units installed in doctors' offices or in hospitals frequently do not receive the necessary servicing required for proper operation thereof. This shortcoming has resulted in a search for methods of transmitting the data digitally by first converting the analog ECG waveforms into digital form.
It is highly desirable that a vector ECG (i.e. three channels of ECG) be transmitted for analysis. However, the bandwidth of conventional telephone transmission circuits is not sufficiently wide to accommodate a vector ECG without data reduction. With the present invention, the average bit rate of an analog-to-digital-converted vector ECG is reduced sufficiently to allow for digital transmission thereof over a low-grade dial-up telephone line. A method of reducing the average bit rate of digitized ECG signals by use of a second-order digital compression filter followed by a Huffman-type encoder is disclosed in an article by U. E. Ruttiman and H. V. Pipberger entitled "Compression of the ECG by Prediction of Interpolation and Entropy Encoding", IEEE Transactions of Biomedical Engineering, Vol. BME-26, No. 11, pp. 613-623, November 1979. Also, the recording of encoded digitized ECG signals is disclosed in an article by K. L. Ripley and J. R. Cox, Jr. entitled, "A Computer System for Capturing Transient Electrocardiographic Data", Proc. Comput. Cardiol. pp. 439-445, 1976. There, digitized ECG signals are second-differenced using computer software, the second-differenced values are Huffman encoded, again using computer software, and the serial bit stream is stored on disc storage means.